In-cylinder NO_x histories in an indirect injection diesel engine: Comparison between experimental data and model predictions

NO_x concentrations within the cylinder of an operating single-cylinder version of a passenger-car type, indirect-injection diesel engine have been measured using a total cylinder sampling system. This system utilizes an explosively actuated cutter to cut a diaphragm sealing the precombustion chamber. The cylinder charge is then rapidly cooled and diluted as it flows from the cylinder into a sampling bag. Pressure decay rates during blowdown range from 3000 to 6000 MPa s^-1. This should be adequate to quench NO_x reactions within a few crankangle degrees. The system has been used to determine in-cylinder NO_x concentrations for eleven different operating conditions. Engine speeds of 1000 and 1500 RPM and the load range corresponding to equivalence ratios ranging from 0.32 to 0.94 were explored. The influences of fuel injection timing and exhaust gas recirculation were also examined. NO_x concentrations rise rapidly after the onset of combustion and approach exhaust levels late in the expansion stroke. Peak NO_x concentrations measured in the cylinder are as much as 15-20% greater than exhaust NO_x concentrations. The experimental data were compared with model predictions made using MIT's stochastic combustion model for this engine. In general, the modeled results were in good agreement with the experimental results. The major difference was that the model did not predict higher NO_x concentrations in the cylinder than in the exhaust.